Single chip microcomputers are known including external communication ports so that the chip may be connected in a network including for example connection to a host microcomputer for use in debugging routines. Such systems are known in which each of the interconnected microcomputer chips has its own local memory. Such microcomputers may have the operation of the CPU suspended without resetting state on the chip. Alternatively they may undergo a reset operation which resets all circuitry on the chip. In this case boot code is required to resume CPU operation.
It is an object of the present invention to provide an improved microcomputer, and an improved method of operating a microcomputer system, in which external communications are simplified and boot code can be obtained from locations off-chip.